Methods and apparatus for operating paired corotrons of opposite polarity

ABSTRACT

Methods and apparatus for applying opposite polarity, equal magnitude charging currents to a surface to be charged are provided according to the present invention. In accordance with one embodiment of the present invention at least two corotrons are energized by a floating power supply exhibiting substantially constant current characteristics. The positive terminal of the floating power supply is connected to a coronode of one of the at least two corotrons while the negative terminal of the floating power supply is connected to the coronode of another one of such at least two coronodes. Additionally, the shields of each of such at least two corotrons are interconnected through a current limiting impedance so that current flow between the shields of the at least two corotrons is maintained within a selected range whereupon the ion charging current produced by each of the corotrons is maintained at substantially uniform magnitude levels.

United States Patent July 4, 1972 Silverberg 1541 METHODS AND APPARATUSFOR OPERATING PAIRED COROTRONS OF OPPOSITE POLARITY [72] Inventor:Morton Silverberg, Rochester, N.Y. [73] Assignee: Xerox Corporation I[22] Filed: Jan. 21,1971

21 App]. No.: 108,304

[52] US. Cl ..250/49.5 ZC, 250/495 GC [51] ..G03g 15/00 [58] Field ofSearch ..250/49.5 ZC

[56] References Cited UNITED STATES PATENTS 3,244,083 4/1966 Gundlach..250/49.5 ZC 3,122,634 2/1964 King ..250/49.5 ZC

Primary Examiner-Walter Stolwein Assistant Examiner-C. E. ChurchAttorney-James J. Ralabate, John E. Beck, Laurence A. Wright and Marn &Jangarathis [57] ABSTRACT Methods and apparatus for applying oppositepolarity, equal magnitude charging currents to a surface to be chargedare provided according to the present invention. In accordance with oneembodiment of the present invention at least two corotrons are energizedby a floating power supply exhibiting substantially constant currentcharacteristics. The positive terminal of the floating power supply isconnected to a coronode of one of the at least two corotrons while thenegative terminal of the floating power supply is connected to thecoronode of another one of such at least two coronodes. Additionally,the shields of each of such at least two corotrons are interconnectedthrough a current limiting impedance so that current flow between theshields of the at least two corotrons is maintained within a selectedrange whereupon the ion charging current produced by each of thecorotrons is maintained at substantially uniform magnitude levels.

9 Claims, 1 Drawing Figure PATENTEDJUL 4 m2 3. 675,01 1

INVESTOR.

Morton Silverberg ATTORNEYS METHODS AND APPARATUS FOR OPERATING PAIREDCOROTRONS OF OPPOSITE POLARITY This invention relates to electrostaticcharging apparatus and more particularly to improved methods andapparatus for applying uniform, equal and opposite corona chargingcurrents to a surface to be charged such as those conventionallyemployed in electrophotographic processes and the like.

In U.S. Letters Pat. No. 3,244,083, as issued to R.W. Gundlach on Apr.15, 1966 and assigned to the Xerox Corporation, there are disclosednovel methods and apparatus for applying a charge to a photoreceptorstructure having an electrostatic image formed thereon so that suchoperations as image transfer or tacking and stripping can beaccomplished Without distortion or destruction of the electrostaticimage formed on the photoreceptor, As set forth in US. Letters Pat. No.3,244,083, supra, the methods and apparatus disclosed thereinessentially contemplate modes of corotron operation wherein at least twocorotrons are energized by a floating power supply. The shields of thetwo corotrons utilized are interconnected and the positive terminal ofthe power supply utilized is connected to a coronode of one of thecorotrons while the negative terminal of such power supply is connectedto the coronode of the other corotron. Since no point in the resultingcorotron circuit configuration is grounded, it can be demonstrated thatat any given instant of time the positive ion charging current deliveredto a common moving surface, such as a selenium drum, by the positivelyconnected corotron must be exactly equal to the negative ion currentdelivered to such moving surface by the negatively connected corotron.The mode of operation thus taught in US. Letters Pat. No. 3,244,083,supra, is highly advantageous because in image transfer operations ortacking and stripping operations, if it is assumed that a neutralizingcharging current which is equal in magnitude but opposite in polarity tothat relied upon in the transfer of an image to a moving surface or inthe tacking portion of an operation is utilized upon completion of thetransfer or tacking step, it will be immediately appreciated by those ofordinary skill in the art that the medium to which such image has beentransferred or the medium to be stripped may readily be removed, due toits electrostatically neutral state, from the surface on which the imagewas initially formed or on which the medium was tacked without anysparking or other deleterious electrostatic effects occurring. This ishighly desirable because sparking and the like will distort or destroythe image present on a moving surface such as a selenium drum and hencethe avoidance of such distorting phenomena would allow subsequent imagetransfers from the same moving surface without necessitating arepetition of the entire electrophotographic process relied upon in theformation of the initial image for each transfer desire.

Unfortunately, in the operation of the invention disclosed in the aboveidentified patent, the operational advantages theoretically predicted bythe use of equal and opposite charging currents in image transfer ortacking and stripping operations have not been fully realized becausealthough equal and opposite charging currents are delivered at eachinstant of time from the positively and negatively connected corotronsto a common moving surface, the same portions of such moving surface donot receive equal and opposite charging cur rents and accordingly arenot rendered electrostatically neutral whereby deleterious electrostaticeffects do occur during the separation or stripping of the imagetransfer material or tacked medium. Detailed study of the operation ofthe electrostatic charging methods and apparatus disclosed in US.Letters Pat. No. 3,244,083, supra, has indicated that the problem areaof operation which precludes the actual operation of the chargingapparatus taught therein from closely approximating the theoreticallypredicted results is that even though at any given instant the chargingcurrent produced by the positively and negatively connected corotronsmust be equal and opposite, they need not be uniform over apredetermined time interval. Thus, as different discrete portions of thesurface to be charged are in a charging relationship with the positivelyand negatively connected corotrons at a given instant, there is norequirement in the teaching of US. Letters Pat. No. 3,244,083, supra,that the negatively connected corotron will apply charging current to adiscrete portion of a surface to be charged which is equal in magnitudeto the positive charging current received by such discrete portion ofthe surface at a preceding instant when such discrete portion of thesurface to be charged was in a charging relationship with the positivelyconnected coronode. Therefore, the practical problems associated withcharging a surface in relative motion with two displaced chargingstations, as represented by the positively and negatively connectedcorotron configuration taught in US. Letters Pat. No. 3,244,083, supra,precludes the actual neutralizing results achieved in the use ofapparatus employing the teaching therein from approaching thetheoretically predicted results because the charging current produced byeach of the corotrons need not be maintained at uniform levels. Thus,simply stated the problem is that while the two ion currents produced atthe oppositely connected corotrons must be equal in magnitude, suchcurrents do not have to be constant or uniform when the teachings of115. Letters Pat. No. 3,244,083, supra, are employed.

In the operation of corotron devices, as stated in my copendingapplication Ser. No. 108,302, filed on an equal date herewith, theactual ion current produced at the coronode is a function of both thevoltage applied between the coronode and the surface to be charged andthe current flowing in the shield. Furthermore, as will be apparent fromthe above, the ion charging current delivered from the coronode to asurface to be charged is also a function of the potential level of thesurface to be charged as seen by the coronode. Thus, in a multiplecorotron charging configuration such as taught in US. Letters Pat. No.3,244,083, supra, not only do the magnitude variations charging currentsproduced by the positively and negatively connected corotronconfigurations result in inaccurate levels in the magnitude of theneutralizing charge applied by the second corotron of a pair, butinaddition thereto, the variations in the charging current produced bythe first corotron of such pair will result in charge variations on thesurface of the medium being charged and these charge variations thuscaused by the first corotron will result in further variations in thecharging current delivered by the second corotron of the pair due to thevariations in charge seen by the coronode of such second corotron. Thus,the

' variations in charging current produced in a multiple corotronconfiguration such as taught in US. Letters Pat. No. 3,244,083, supra,have a cumulative effect from the standpoint of the inability to achievethe delivery of a charging current from the second corotron of the pairhaving an appropriate magnitude to precisely neutralize the tacking orimage transfer charge applied by the oppositely connected firstcorotron; and, it is this cumulative effect which results in asubstantial disparity in the charging current delivered by thepositively and negatively connected corotron configurations taught bythe subject patent. This disparity, it should be noted, is particularlypronounced at the leading and trailing edges of the image area on thesurface to be charged and acts to preclude the removal or stripping ofthe image transfer or tacked member from the surface to be charged withessentially no net charge associated therewith.

Therefore, it is a principal object of this invention to provide methodsand apparatus for operating paired corotrons which are oppositelyconnected to a common supply in a manner such that substantially uniformcharging currents which are equal in magnitude but opposite in polarityare produced thereby. Various other objects and advantages of thepresent invention will become clear from the following detaileddescription of an exemplary embodiment thereof, and the novel featureswill be particularly pointed out in conjunction with the claims appendedhereto.

In accordance with the teachings of the present invention methods ofcorotron operation and apparatus therefor are provided wherein at leasttwo corotrons are energized by a floating power supply exhibitingsubstantially uniform current characteristics, the positive terminal ofthe floating power supply is connected to a coronode of one of said atleast two corotrons while the negative terminal of said floating powersupply is connected to a coronode of another one of said at least twocorotrons, the shields of each of said at least two corotrons areinterconnected through current limiting resistor means so that currentflow between the shields of said at least two corotrons is maintainedwithin a selected range whereupon the ion charging current produced byeach of said corotrons is maintained at substantially uniform magnitudelevels.

The invention will be more clearly understood by reference to thefollowing detailed description of an exemplary embodiment thereof inconjunction with the accompanying drawing in which:

The FIGURE is a schematic representation of an exemplary embodiment ofpaired corotron charging apparatus in accordance with the teachings ofthe instant invention.

Referring now to the drawing, there is shown a schematic representationof an exemplary embodiment of paired corotron charging apparatus inaccordance with the teachings of the present invention. The embodimentof the present invention illustrated in the Figure comprises first andsecond corotrons l and 2 connected to a floating power supply 3 and theresulting paired corotron charging apparatus is disposed in a chargingrelationship with a medium 4 to be charged. The structure of the firstand second corotrons 1 and 2 shown in the Figure may be entirelyconventional and comprises a coronode 5 and 6 disposed in apredetermined relationship with the medium 4 to be charged and a shield7 and 8, respectively, associated with its respective coronode 5 and 6.The coronodes 5 and 6, whose end views are illustrated in the Figure,may take the form of a fine strand of conductive wire disposedlongitudinally across the width of the medium 4 to be charged and spacedan appropriate distance therefrom. The coronode wires 5 and 6 wouldnormally be insulated from their respective shields 7 and 8 positionedthereabout by terminal means (not shown) associated with the endportions thereof in the well-known manner shown for instance in US.Letters Pat. No. 3,244,083, supra, Although only a single coronode 5 and6 has been illustrated for each of the corotrons 1 and 2, respectively,it will be apparent to those of ordinary skill in the art thatmulti-coronode corotrons may be relied upon in applications where it isdesired to increase the uniformity of the charging current along thewidth of the medium to be charged. Similarly, the shields 7 and 8 maytake the conventional form of conductive material having a selectedconfiguration disposed in the longitudinal direction in a predeterminedrelationship with the coronodes 5 and 6, respectively, and fixedlypositioned at a selected distance therefrom. Although any of thewell-known shield configurations illustrated or discussed for instancein US. Letters Pat. No. 2,879,395, to L.E. Walkup and assigned to theXerox Corporation, may be relied upon in the practice of the instantinvention, half-round shield configurations have been illustrated asexemplary; it being noted that such shield configurations are desireablebecause they tend to keep the coronodes associated therewith cleanerduring the continued operation thereof and maintain the charging currentproduced by a given coronode more uniform with time.

The coronode 5 of the first corotron l is connected through theimpedance means R, to one terminal of the high potential power source 3while the coronode 6 of the second corotron 2 is connected to anopposite polarity terminal of the high potential power source 3. Thehigh potential power source 3 may take the form of a DC supply havingthe positive terminal thereof connected to one of the coronodes 5 or 6while the negative terminal thereof is connected to the other coronode 6or 5. Alternatively, an AC or pulsed DC source of potential may berelied upon depending on the nature of the charging operation selected.In any event, neither of the terminals of the high potential powersource 3 is grounded and preferably,

the high potential power source 3 is insulated in the manner describedin US. Letters Pat. No. 3,244,083, supra, so that the high potentialpower source 3 is maintained in a clearly floating posture with respectto ground. Additionally, the high potential power source 3 shouldpreferably exhibit a peak-topeak tenninal voltage in the range ofapproximately (9-20KV) and be capable of supplying current on acontinuous basis in the range of (-400 ya). Although the preciseelectronic configuration relied upon for the high potential power source3 forms no part of the present invention, any of the high potentialpower source configurations mentioned in U.S. Letters Pat. No.3,244,083, supra, clearly may be relied upon. The impedance means R,interposed between one ter minal of the high potential power source 3and the coronode 5 may take the form of an ordinary resistor whichexhibits a I value of resistance which is large compared to the dynamicimpedance of the corotrons l and 2 so that regardless of any variationin the dynamic impedance of the corotrons l and 2, the load seen by thehigh potential power source 3 is essentially constant and a substantialportion of the voltage drop takes place across the resistor R,. As thedynamic impedance exhibited by conventional corotrons, such corotrons 1and 2 presently under discussion herein, is usually in a range of from0.25-10 megohms, the value of resistance manifested by the impedancemeans R, will be in a range of from (10-100 MQ). Therefore, it will beappreciated by those of ordinary skill in the art that the impedancemeans R, and the high potential power source V, act in combination toform a constant current source for supplying a substantially constantcurrent to the coronodes 5 and 6. As an alternative to using thecombination of a high potential power supply and a large valueresistance in series therewith, it will be obvious to those of ordinaryskill in the art that a highly regulated power supply designed tofunction as a constant current source could be substituted therefor.Such constant current supplies are well known and may be regulatedeither electronically or by conventional loading techniques.

The shields 7 and 8 of each of the corotrons l and 2, respectively, areinterconnected according to this embodiment of the present inventionthrough a current limiting impedance means R The function of the currentlimiting impedance means R is to ensure that the current flowing betweenthe shields 7 and 8 is maintained above a value of approximately 1ua/in. of corotron length so that ion charging current from the coronodemay be initiated at a lower threshold value of voltage between thecoronodes and the surface to be charged while such current limitingimpedance means R additionally acts to limit the current flowing betweenshields 7 and 8 below a predetermined level so that corona chargingtakes place in a highly efiicient manner since the voltage levelautomatically established across the impedance means R prevents all buta small portion of the ion charging current produced by the coronodes 5and 6 from being diverted to the shields 7 and 8. The manner in whichcurrent flow in the shield above a predetermined minimum value reducesthe threshold value of voltage necessary between a coronode and asurface to be charged to initiate ion charging current flow from thecoronode as well as the manner in which such shield current establishesa highly efficient range of corotron operation by precluding ioncharging current from being diverted to the shield is set forth indetail in my copending application Ser. No. 108,302, filed on an equaldate herewith. Therefore, for the purpose of the description of theinstant invention, it is sufficient to appreciate that when a currentflow equal to or greater than one microampere per inch of corotronlength is established between the shields 7 and 8, the threshold valueof voltage at which corona charging current is produced at the coronodes5 and 6 is substantially reduced while the value of the impedance meansR and the voltage drop thereacross precludes corona charging currentfrom being diverted to the shield thereby requiring the vast majority ofthe ion charging current produced by the coronodes 5 and 6 to bedelivered to the surface to be charged. As the current flow between theshields 7 and 8 is increased, improved charging characteristics areobserved until the shield current reaches a value of approximatelyua/in. at which point no further improvement is observed in thecharacteristics. Therefore, the value of the impedance means R ispreferably selected at a value such that the resistance exhibitedthereby maintains the current flow between shields 7 and 8 at a value inthe range of from 1-10 ua/in. of corotron length. The value of theresistance to be exhibited by the impedance means R may beexperimentally achieved by varying the impedance thereof until thedesired current flows in the shield loop and for this reason theimpedance means R is illustrated as a variable resistance device in theFigure. Alternatively, the value of resistance to be exhibited by thecurrent limiting impedance R may be calculated using conventionaltechniques as the value of the impedance means R is known and thedynamic impedance exhibited by the first and second corotrons 1 and 2may be determined. Regardless of the technique employed, the value ofresistance exhibited by the current limiting impedance means R willordinarily fall within the range of (100-500 MO.) depending upon theother parameters exhibited by the embodiment of the inventionillustrated in the Figure. Thus it is seen that according to theembodiment of the instant invention, the basic corotron configurationdiscussed in US. Letters Pat. No. 3,244,083, supra, has been altered sothat a floating constant current source is relied upon to supplycharging current to each of the coronodes 5 and 6 while a currentlimiting impedance means R is interposed in the connection between theshields 7 and 8 so that current flow therebetween is maintained withinpredetermined limits.

The resulting corotron configuration illustrated in the Figure isdisposed in a charging relationship with a medium 4 to be charged.Although the charging configuration illustrated in the Figure may berelied upon for tacking and stripping operations involving anyinsulating material or in image transfer operations as aforesaid, animage transfer application of the instant invention has been depicted inthe Figure in order to illustrate an exemplary mode of operation of thepresent invention. Accordingly, in the Figure, the medium 4 will heretake the form of a transfer member to which a developed electrostaticimage is to be transferred. The medium 4 may thus comprise a sheet, webor drum formed of suitable transfer material such as paper, plastic orany of the other well known materials conventionally employed astransfer materials and is disposed proximate to a photoreceptor 9 havinga developed electrostatic image formed thereon. The photoreceptor 9 maytake the form of a sheet, web or drum formed of suitable photoconductivematerials such as selenium, dispersions thereof or the like which may beovercoated on a conductive or nonconductive substrate. The materialsrelied upon in the formation of the photoreceptor 9 form no part of theinstant invention and thus it is here sufficient to state that thephotoreceptor 9 may take any of the well-known forms of structures whichadmit of the formation of a latent electrostatic image upon theapplication of electrophotographic processes thereto. A partial listingof suitable photoconductive materials which may be used in the formationof the photoreceptor 9 may be found in my copending application Ser. No.108,302 filed on an equal date herewith.

The photoreceptor 9, as aforesaid, has a developed electrostatic imagethereon which ordinarily would be formed by conventionalelectrophotographic processes not associated with the corotron chargingconfiguration depicted in the Figure. For instance, a latentelectrostatic image would be initially formed by subjecting thephotoreceptor to conventional electrophotographic processing steps suchas sensitizing the photoreceptor by the application of an electrostaticcharge thereto and selectively exposing the photoconductive portion ofsuch photoreceptor 9 to electromagnetic radiation from a light and darkpattern representing the image to be formed. Thereafter, thephotoreceptor 9 having the thus formed latent electrostatic imagethereon could be developed or further charged and then developed torender the latent electrostatic image formed viewable. The developmentof the electrostatic latent image could be accomplished by suchwell-known techniques as cascade development, as described in Us.Letters Pat. No. 2,618,551, to L. E. Walkup, where a mixture of tonerpaiticles are applied to the surface and will selectively adhere to thesurface area of the photoreceptor 9 in accordance with the latentelectrostatic image present thereon or alternatively any of the otherwell-known development techniques such as powder cloud development,brush development, loop development, magnetic development or the likecould be used. Furthermore, the foregoing processing steps or otherelectrophotographic processing steps well known to those of ordinaryskill in the art could be carried out in the form of a continuousprocess, as would be the case if the photoreceptor 9 was in the form ofa drum or endless web having apparatus for carrying out each of saidsteps disposed about the periphery thereof, or alternatively as a seriesof discrete steps as would be the case if the photoreceptor 9 was in theform of a sheet or web which was conveyed past a series of processingstations. Thus, regardless of the electrophotographic process reliedupon, the photoreceptor 9 having a developed latent image thereon isprovided prior to the image transfer step depicted in the Figure.

After the photoreceptor 9 has a latent electrostatic image formedthereon and such latent electrostatic image is developed, the transfermember 4 is brought into a proximate relationship therewith so that thetransfer member 4 may be charged whereupon the toner particles will betransferred from the surface of the photoreceptor 9 to the surface ofthe transfer member 4 and thereafter an equal and opposite charge isapplied to transfer member 4 in accordance with the teachings of thepresent invention so that such transfer member may be stripped from thephotoreceptor 9. In the Figure, the latent electrostatic image formed onthe photoreceptor 9 is indicated by the positive charges proximate tothe surface thereof and the dots intermediate the adjacent surface ofthe photoreceptor 9 and transfer member 4 indicate toner particlesadhered to appropriate portions of such surface areas. Although in theFigure positive charges have been relied upon to indicate the chargepattern associated with the latent image formed and toner particles havebeen indicated as attracted to such positive charges; it will be obviousto those of ordinary skill in the an that a negative charge pattern maybe relied upon in the formation of the latent image by altering orreversing the polarity of the initial sensitizing charge utilized in theelectrophotographic process and either positively or negatively chargedtoner particles may be utilized depending upon whether or not a positiveor negative image configuration on the photoreceptor is desired. Thetransfer member 4 is brought into contact with the surface of thephotoreceptor 9 just .prior to the transfer operation and the transfermember 4 and the photoreceptor 9 are maintained in relative motion withrespect to the first and second corotrons l and 2 illustrated in theFigure. This may be accomplished by any of the techniques well known tothose of ordinary skill in the art, for instance, if the photoreceptor 9takes the form of a rotating drum, the paired corotron configurationillustrated in the Figure would represent a charging station locatedabout the periphery of such drum while the transfer medium could takethe form of a moving web proximate to the portion of the photoreceptordepicted in the Figure and having a supply reel positioned to the rightof the second corotron 2 and a take-up reel located to the left of thecorotron 1. Another obvious way of arriving at the desired result is torely upon a web or sheet form of photoreceptor 9, disposing a web orsheet form of transfer member thereon after the latent electrostaticimage is formed and developed and then either conveying thephotoreceptor 9 having the transfer member 4 thereon past the corotronsl and 2 or alternatively adapting the paired corotron configuration tobe displaced in a direction from left to right across the surface ofthetransfer member 4. Thus, re-

' gardless of the techniques employed, it will be seen that aphotoreceptor 9 having a developed latent electrostatic image thereonand a transfer member 4 proximate to the charged surface thereof areplaced in relative motion and in a charging relationship with the pairedcorotron charging configuration according to this invention in the imagetransfer application of this invention illustrated in the Figure.

The operation of the embodiment of this invention illustrated in theFigure will be set forth below under the assumption that a transfer ofthe developed image from the photoreceptor 9 to the transfer member 4 isto be accomplished and thereafter the transfer member 4 is to bestripped from the photoreceptor 9. Accordingly, the polarities of theion charging current discussed below will be those appropriate foraccomplishing image transfer and stripping for the toner imageillustrated; however, it will be readily appreciated by those ofordinary skill in the art, that either of the coronodes 5 and 6 mayproduce a positive or negative ion charging current depending upon thepolarity of the connection of the high potential power source 3selected. Furthermore, for the purposes of the description of thisinvention which follows, it will be assumed that the photoreceptor 9having the transfer member 4 disposed thereon is adapted fordisplacement from right to left with respect to the first and secondcorotrons l and 2.

In the operation of the embodiment of this invention illustrated in theFigure, negative toner particles are relied upon in the development ofthe latent electrostatic image on the photoreceptor 9 and hence a directtransfer operation will require that positive charging be carried out bythe second corotron 2 while negative charging is carried out by thefirst corotron 1. Therefore, where these conditions obtain, the highpotential power source 3 will be connected in such manner that thepositive terminal thereof is connected to the coronode 6 while thenegative terminal of the high potential power source 3 is connected tothecoronode 5. With the high potential power source 3 connected in theabove mentioned polarity the second corotron 2 will deliver positive ioncharging current to the surface of thetransfer member 4 while the firstcorotron 1 will deliver negative ion charging current to the surface ofthe transfer member 4. Furthermore, since no point in the circuitbetween the first and second corotrons l and 2 and the high potentialpower source 3 is grounded, the positive ion charging current deliveredby the second corotron 2 to the moving surface of the transfer member 4must be exactly equal at any given instant to the negative ion chargingcurrent delivered to such surface by the negatively connected firstcorotron l, in the same manner as set forth in U.S. Letters Pat. No.3,244,083, supra. Additionally, as the current path established from thecoronode 6 through the interconnected shields 8 and 7 to the coronode 5is effectively in parallel with an ion charging current path residingbetween each of the coronodes 5 and 6 and the surface to be charged, asrepresented by the transfer member 4, it will be appreciated that theion charging current I, delivered to the surface to he charged is givenby the relationship that:

i r I): a wherein 1,, is the current applied to the coronodes 5 and 6while I, is the current flowing in the shield. The current I, applied tothe coronodes 5 and 6 is supplied to the coronodes 5 and 6 by theconstant current source formed by the high potential power source 3 inserial combination with the impedance means R or a suitable alternativetherefor as mentioned above. Therefore, the value of the current 1applied to the coronodesS and 6 is essentially a constant with time aschanges in the dynamic impedance exhibited by the first and secondcorotrons l and 2 due to ambient conditions will be small with respectto the resistance value of the impedance means R as seen by the highpotential power source. Thus, the value of current I, applied to thecoronodes 5 and 6 is in a range of from 50-400 pa) and is maintained atessentially a constant value. In addition, as the shield current I,present in the current path formed by the interconnection of the shields7 and 8 through the current limiting impedance means R is small, beingpreferably limited in value as aforesaid to a range of from l-l ya/in,of corotron and in any event since the current limiting impedance meansR exhibits a value which is exceedingly large, i.e., -500 MR, as statedabove, with respect to the dynamic impedance of the first and secondcorotrons 1 and 2 so that the impedance exhibited by the shield currentpath will be essentially constant despite changes in the dynamicimpedance of the corotrons l and 2; the value of current I, flowing inthe current path between the shields l and 2 will be small andessentially constant in magnitude. Therefore, as both the value ofcurrent I,, applied to the coronodes 5 and 6 and the current flow 1,between the shields 7 and 8 are constant in value, it will be seen thatthe value of charging current I, delivered to the surface to be chargedyielded by the expression:

will be essentially constant in value. Accordingly, it will be seen thatthe paired corotron charging configuration illustrated in the Figure notonly applies charging current of equal magnitude and opposite polarityto the surface to be charged at each instant of the operation, but inaddition thereto, the value of charging current delivered to the surfaceto be charged by the first and second corotrons 1 and 2 will beessentially constant. This means that regardless of the spacing betweenthe corotrons l and 2, the second corotron 2 will apply a positive ioncharging current to the surface to be charged whose magnitude isconstant in value and the first corotron 1 will apply a negative ioncharging current to the surface to be charge whose magnitude isprecisely equal to the constant value of the positive charging currentdelivered by second corotron 2. Thus, the instant invention allows thepractical results achieved by connecting a pair of corotrons to oppositeterminals of a floating supply to approach the theoretical advantagespredicted because as variations in v the ion charging currents deliveredto a surface to be charged are avoided, exact magnitudes of neutralizingion current will always be supplied to the surface to be charged by thesecond corotron of the pair regardless of the spacing between the pairof corotrons and the time interval between the application of successivecharges. In addition, the maintenance of the current flowing in theshield preferably within a range of of corotron (l-lO #a/in.) is highlydesirable because, as described in more detail in my copendingapplication Ser. No. 108,302, ion current flow may be initiated from thefirst and second corotrons l and 2 with a lower threshold voltageapplied thereto and large portions of the ion charging current producedat the coronodes 5 and 6 may not be diverted to the shield and hence aredelivered to the surface to be charged. This enables the first andsecond corotrons l and 2 to be operated in a highly efficient mannerwhereby sufficiently large magnitudes of charging current are deliveredto the surface to be charged so that regardless of minor charge levelvariations thereon, the potential level associated with the corotron isimposed upon such surface to be charged.

When paired corotron apparatus according to the present invention isemployed in an image transfer operation of the type indicated in theFigure, the negatively charged toner particles initially adhered to thesurface of the photoreceptor 9 due to electrostatic forces of attractionare transferred to the adjacent surface of the transfer member 4 andthereafter the charge on the transfer member 4 is neutralized in themanner detailed below. Prior to receiving charging current from eitherof the first and second corotrons 1 and 2, the transfer member 4disposed on the surface of the photoreceptor 9 is in physical contactwith the toner image at only a few points on the surface thereof due tothe absence of a strong force of attraction between adjacent surfaces ofthe transfer member 4 and photoreceptor 9 while the remaining portionsof the lower surface of the transfer member 4 may be separated from thetoner image on the photoreceptor 9 by an air gap of several microns.When, however, the successive portions of the photoreceptor 9 having thetransfer member 4 disposed thereon are displaced under the secondcorotron 2, which under the conditions assumed above for a negativetoner produces positive charging current, the appropriate portion of thetransfer member 4 in a charging relationship with the second corotron 2will be subjected to a bombardment of positive ions along the widththereof which seek to impose the potential of the second corotron 2 onthe appropriate surface portion of the transfer member 4. The positivecharges applied to the surface of transfer member 4 will migrate to theopposite surface thereof adjacent to the toner image and hence willinduce negative charges in corresponding portions of the conductivebacking or substrate of the photoreceptor 9. The charge on the portionsof the transfer member 4 receiving positive ion charging current fromthe second corotron 2 will produce a very substantial force ofattraction between corresponding portions of the adjacent surfaces ofthe transfer member 4 and the photoreceptor 9 thereby bringing suchportions of the transfer member 4 into intimate contact with theportions of the toner image on the adjacent portions of thephotoreceptor. When these conditions obtain, the field strength betweenthe charged portion of the transfer member 4 and the adjacent portion ofthe photoreceptor 9 will be sufficient to cause most of the negativelycharged toner to be transferred from the photoreceptor 9 to the transfermember 4 as indicated in the Figure for the portions of the transfermember 4 and the photoreceptor 9 directly under and to the left of thesecond corotron 2. This operation will be continued for successiveportions of the transfer member 4 on a continuous or discontinuous basisuntil the entire toner image has been transferred to the transfer member4.

Prior to the invention set forth herein and that disclosed in U.S.Letters Pat. No. 3,244,083, supra, it was conventional to remove thetransfer member 4 from the photoreceptor 9 subsequent to the transfer ofthe toner image by a peeling operation' or the like. Because thetransfer member 4 being formed of paper, plastic or the like isgenerally a relatively fair insulator, the charge thereon tended toremain essentially constant as the transfer member 4 was removed so thatthe potential thereof would increase linearly with separation. When theseparation between the transfer member 4 and the photoreceptor 9 reacheda distance equal to about p), the air gap therebetween would break downresulting in a discharge taking place between the transfer member 4 andthe photoreceptor 9. This discharge would leave, under the conditionsspecified herein, a positive charge on the surface of the photoreceptor9 and substantially destroy or distort the latent electrostatic imageformed thereon so that such image was no longer suitable for secondaryuses if desired.

With the paired corotron apparatus according to the present invention,however, successive portions of the transfer member 4 and thephotoreceptor 9 are brought into a charging relationship with the firstcorotron l subsequent to the charging and image transfer stepaccomplished by the second corotron 2. The first corotron l, asaforesaid, will subject the portions of the transfer member 4successively brought into a charging relationship therewith to anegative charging current whose magnitude is precisely equal to thepositive charging current then being delivered to subsequent portions ofthe transfer member 4 by the second corotron 2. Furthermore, as thefirst and second corotrons l and 2 produce essentially uniform currentsaccording to the teachings of this invention, the negative ion chargingcurrent applied to successive portions of the transfer member 4 will beequal in magnitude to the positive ion charging current applied to thesesame portions of the transfer member 4 by the second corotron 2 duringthe preceding toner image transfer step. Thus, as successive portions ofthe transfer member pass beneath the first corotron l, the chargethereon will be precisely neutralized in the well-known manner wherebythe electrostatic relationship which is present between the transfermember 4 and the photoreceptor 9 is restored to that which obtainedprior to the charging operation carried out by the second corotron 2which resulted in image transfer. Therefore, subsequent to the chargingoperation carried out by the first corotron l, the transfer member 1 maybe easily removed from the photoreceptor 9 without discharge occurringtherebetween so that attendant destruction or distortion of the latentelectrostatic image on the photoreceptor 9 is avoided. Thus, the latentelectrostatic image on the photoconductor 9 may be reused or retainedfor possible later use,

Therefore, it is seen that the instant invention provides methods andapparatus for operating paired corotrons which are oppositely connectedto a common power supply in a manner such that substantially uniformcharging currents which are equal in magnitude but opposite in polarityare obtained. Accordingly, the present invention allows image transferor tacking and stripping operations to be performed while thedeleterious electrostatic discharge normally attending the removal ofthe tacked or image transfer member is avoided.

Although the present invention has been disclosed in conjunction with aspecific exemplary embodiment thereof, various alternatives andmodifications to the specific structure set forth herein will be obviousto those of ordinary skill in the art. For instance, multiple coronodecorotron structure could readily be substituted for the single coronodecorotrons illustrated while corotron configurations employing multiplepairs of corotrons are readily available. Additionally, due to the highefficiency in the manner in which the corotrons disclosed herein areoperated, substantially larger values of delivered charging current areavailable and hence copying apparatus employing the instant inventionmay be operated at speeds which are higher than previously available oralternatively if it is desired to retain conventional operating speeds,lower value potential sources may be used in apparatus according to theteachings of the invention contained herein. Furthermore, as will beappreciated by those of ordinary skill in the art, the specific valuesand circuit parameters set forth herein are to be viewed only asexemplary and not in a limiting sense.

While this invention has been described in connection with an exemplaryembodiment thereof, it will be understood that many modifications willbe readily apparent to those of ordinary skill in the art; and that thisapplication is intended to cover any adaptations or variations thereof.Therefore, it is manifestly intended that this invention be only limitedby the claims and the equivalents thereof.

What is claimed is:

1. In a charging circuit for applying opposite polarity ion chargingcurrents to a surface to be charged, said charging circuit includingfirst and second corotron means disposed in a charging relationship withsaid surface to be charged and each of said first and second corotronmeans including coronode means for producing said ion charging currentand shield means associated with said coronode means, wherein theimprovement comprises:

constant current source means connected at a first polarity tenninalthereof to a coronode means of one of said first and second corotronmeans and connected at a second polarity terminal thereof to saidcoronode means of another of said first and second corotron means, saidconstant current source means being isolated from any external referencepotential other than those associated with connections to said first andsecond polarity terminals thereof; and

means for interconnecting said shield means of each of said first andsecond corotron means.

2. The improved charging circuit according to claim 1 wherein said meansfor interconnecting said shield means of each of said first and secondcorotron means comprises current limiting impedance means connectedbetween said shield means of each of said first and second corotronmeans.

3. The improved charging circuit according to claim 2 wherein saidcurrent limiting impedance means exhibits a value of resistance which islarge compared to the dynamic impedance exhibited between said coronodemeans and said shield means of each of said first and second corotronmeans.

4. The improved charging circuit according to claim 3 wherein saidcurrent limiting impedance means exhibits a value of resistancesufficient to limit current flow therethrough to a small fraction of thecurrent value applied to each of said coronode means by said constantcurrent source means.

5. The improved charging circuit according to claim 4 wherein saidconstant current source means comprises high potential power supplymeans serially connected to high impedance circuit means, said seriallyconnected high impedance circuit means exhibiting a value of resistancewhich is substantial compared to the dynamic impedance exhibited by saidfirst and second corotron means.

6. The improved charging circuit according to claim 5 wherein saidcurrent limiting impedance means exhibits a value of resistancesufficient to limit current flow therethrough to a range of one to tenmicroamperes per inch of corotron length.

7. The improved charging circuit according to claim 6 wherein saidcurrent limiting impedance means exhibits a value of resistance equal toor greater than the resistance exhibited by said serially connected highimpedance circuit means.

8. A method of applying opposite polarity ion charging currents to asurface to be charged comprising the steps of:

disposing first and second corotron means in an appropriate chargingrelationship with said surface to be charged, each of said first andsecond corotron means including coronode means for producing said ioncharging currents and shield means associated with said coronode means;

connecting said coronode means of one of said first and second corotronmeans to a terminal of a constant current source means exhibiting afirst polarity and connecting said coronode means of another one of saidfirst and second corotron means to another terminal of said constantcurrent source means exhibiting a second polarity; and

interconnecting said shield means of said first and second corotronmeans.

9. The method of applying opposite polarity ion charging currents to asurface to be charged according to claim 8 wherein the step ofinterconnecting said shield means of said first and second corotronmeans is accomplished by connecting current limiting impedance meansbetween said shield means of said first and second corotron means.

1. In a charging circuit for applying opposite polarity ion chargingcurrents to a surface to be charged, said charging circuit includingfirst and second corotron means disposed in a charging relationship withsaid surface to be charged and each of said first and second corotronmeans including coronode means for producing said ion charging currentand shield means associated with said coronode means, wherein theimprovement comprises: constant current source means connected at afirst polarity terminal thereof to a coronode means of one of said firstand seCond corotron means and connected at a second polarity terminalthereof to said coronode means of another of said first and secondcorotron means, said constant current source means being isolated fromany external reference potential other than those associated withconnections to said first and second polarity terminals thereof; andmeans for interconnecting said shield means of each of said first andsecond corotron means.
 2. The improved charging circuit according toclaim 1 wherein said means for interconnecting said shield means of eachof said first and second corotron means comprises current limitingimpedance means connected between said shield means of each of saidfirst and second corotron means.
 3. The improved charging circuitaccording to claim 2 wherein said current limiting impedance meansexhibits a value of resistance which is large compared to the dynamicimpedance exhibited between said coronode means and said shield means ofeach of said first and second corotron means.
 4. The improved chargingcircuit according to claim 3 wherein said current limiting impedancemeans exhibits a value of resistance sufficient to limit current flowtherethrough to a small fraction of the current value applied to each ofsaid coronode means by said constant current source means.
 5. Theimproved charging circuit according to claim 4 wherein said constantcurrent source means comprises high potential power supply meansserially connected to high impedance circuit means, said seriallyconnected high impedance circuit means exhibiting a value of resistancewhich is substantial compared to the dynamic impedance exhibited by saidfirst and second corotron means.
 6. The improved charging circuitaccording to claim 5 wherein said current limiting impedance meansexhibits a value of resistance sufficient to limit current flowtherethrough to a range of one to ten microamperes per inch of corotronlength.
 7. The improved charging circuit according to claim 6 whereinsaid current limiting impedance means exhibits a value of resistanceequal to or greater than the resistance exhibited by said seriallyconnected high impedance circuit means.
 8. A method of applying oppositepolarity ion charging currents to a surface to be charged comprising thesteps of: disposing first and second corotron means in an appropriatecharging relationship with said surface to be charged, each of saidfirst and second corotron means including coronode means for producingsaid ion charging currents and shield means associated with saidcoronode means; connecting said coronode means of one of said first andsecond corotron means to a terminal of a constant current source meansexhibiting a first polarity and connecting said coronode means ofanother one of said first and second corotron means to another terminalof said constant current source means exhibiting a second polarity; andinterconnecting said shield means of said first and second corotronmeans.
 9. The method of applying opposite polarity ion charging currentsto a surface to be charged according to claim 8 wherein the step ofinterconnecting said shield means of said first and second corotronmeans is accomplished by connecting current limiting impedance meansbetween said shield means of said first and second corotron means.